Structural sandwich composites—which are sandwich-like arrangements of a relatively low-density core material bonded between comparatively thin, high-strength and high-stiffness skins—are used in a wide variety of applications that require lightweight, yet structurally strong materials. To name but a few applications, structural sandwich composites are used in boating, construction, aviation, rapid transit, and recreational vehicles. Structural sandwich composites are useful because of their high strength and low weight per unit area. When bonded between skins, the low-density core provides a large strength and stiffness enhancement over the skins alone, but adds only a comparatively small weight. To illustrate the benefits of structural sandwich composite construction, consider that dividing a material (e.g., aluminum or fiberglass) into two skins and bonding a core material that is twice the original material's thickness in between them, results in a composite having a stiffness 7 times greater and a strength 3.5 times greater than the original material's while having a density only 1.03 times that of the original material. ANDREW C. MARSHALL, COMPOSITE BASICS 3-1 (5th ed. 1998).
How well a sandwiched core material functions in real-world applications can be predicted from laboratory measurements of its compression strength and modulus, tensile strength and modulus, and shear strength and modulus.
The properties of the core material are of great importance. Desirable properties include high strength, low density, rigidity, high chemical and heat resistance, and low cost. The most common core materials are wood, honeycomb structures, and foams comprising both thermoplastic and thermosetting compositions. Wood core materials suffer from variations in properties and are susceptible to fungal decay, especially in marine use. Honeycomb cores are of an open structure, i.e., comprised of contiguous, connected, and/or interlocked cells, and are typically constructed from rigid materials, such as thermoplastics, fiberglass, aluminum, and stainless steel. While honeycomb-core materials provide strong, high-quality, chemically resistant composites, they are difficult to manufacture. The connected nature of the cells precludes composite manufacture by vacuum-mediated resin techniques because the vacuum draws the resin into the individual cells. Furthermore, honeycomb cores are not suitable for marine applications because a crack in the composite skin can lead to the entire composite filling with water. Closed-cell thermoplastic or thermosetting foams avoid some of these problems, but generally are thermally and chemically sensitive; thus, their composites cannot be used in certain higher-temperature applications. A further disadvantage of thermoplastic- or thermosetting-foam core materials is that certain resin-type adhesives can significantly degrade them, both chemically and via the heat evolved during the cure process.
Skins can be attached to core materials by a variety of methods. One of the most popular methods, because of the high shear strength of the resulting composite, is bonding the skins to the core with a resin (the resin-cure method). The resin-cure method provides structural sandwich composites with excellent skin-core adhesion and delamination resistance. In the resin-cure method, an uncured resin is applied to the contacting surfaces, the core and the skins are contacted, and bonding results upon resin cure. Often, a reinforcing material such as a glass-fiber fabric or mat is combined with the uncured resin to improve strength and stiffness in the resulting joint. During resin cure, substantial heat is generated.
Vacuum-bagging and vacuum-injection-molding techniques are used commercially to introduce the resin between the skins and suitable cores see, for example, U.S. Pat. No. 6,159,414 (issued May 18, 1999); U.S. Pat. No. 5,316,462 (issued May 31, 1994); and U.S. Pat. No. 5,834,082 (issued Nov. 10, 1998). In this process, vacuum is used to draw the uncured resin between the core and skin. Advantageously, the vacuum removes resin fumes as well as shields the uncured resin from air.
With some core materials, however, such as honeycomb structures, vacuum-mediated resin application is difficult or impossible. And unfortunately, in these cases, open-air resin application is proscribed because the hazardous resin fumes are not contained and resin curing can be inhibited by air and moisture. Thus, thermoplastic or thermosetting foams are ideal in that they do not suffer from the biodegradability of wood cores and are amenable to vacuum-mediated resin application. But a serious drawback with thermoplastic- or thermosetting-foam cores is that the heat evolved during resin cure and the chemically corrosive properties of the resin can degrade them, resulting in weaker composites.
Thermoplastic polyester resins, such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) that have been pre-treated with branching agents (hereinafter “branched polyesters”) yield closed-cell foams having excellent strength and mechanical properties, low density, and high chemical and thermal resistance. The branching agents, which have multiple chemical-reaction sites, function by chemically condensing two or more polyester chains (“branching”). This branching gives the pre-foam polyester melt viscoelastic properties more suitable for foaming, leading to higher quality foams. Polyester foams, prepared from branched polyesters, such as branched polyethylene terephthalate, have been disclosed in U.S. Pat. No. 5,000,991 (issued Mar. 19, 1991); U.S. Pat. No. 5,229,432 (issued Jul. 20, 1993); U.S. Pat. No. 5,340,846 (issued Aug. 23, 1994); U.S. Pat. No. 5,362,763 (issued Nov. 8, 1994); U.S. Pat. No. 5,422,381 (issued Jun. 6, 1995); U.S. Pat. No. 5,679,295 (Oct. 21, 1997); U.S. Pat. No. 5,681,865 (Oct. 28, 1997); U.S. Pat. No. 6,342,173 (issued Jan. 29, 2002), each of which eight patents are hereby incorporated by reference herein. These foams are closed-cell structures with low densities, excellent mechanical properties, and high thermal and chemical resistance. Regrettably, because the process used for their manufacture leads to irregular surfaces, such foams make mediocre to poor core materials. The irregular surfaces promote weak bonding to the composite skin and wide cell-size distribution and, therefore, poor mechanical properties. To explain more fully, polyesters are generally foamed by extruding a pressurized mixture of a branched-polyester melt and a volatile, organic expanding or “blowing agent” through an annular or slit die. Upon entering ambient pressure, the blowing agent evaporates and the polyester foams. This process suffers in that if the die opening size surpasses a critical limit, extruder pressure cannot be maintained. Furthermore, as the die opening is enlarged to the size required for use as a core material, blowing-agent evaporation throughout the material becomes non-uniform leading to erratic cell-size distribution, oversized cells, and an irregular surface.
Coalesced-strand polyester foams are more suitable as core materials because they can be produced in thicker size with a uniform distribution of small cells. Coalesced-strand polyester foams are disclosed in U.S. Pat. No. 5,475,037 (issued Dec. 12, 1995). Generally, coalesced-strand thermoplastic foams are prepared by melting a thermoplastic resin, mixing the melt with a blowing agent, and extruding the resulting gel through a multi-orifice die. The orifices are so arranged such that some contact between adjacent strands occurs during foaming, and the contacting strand surfaces adhere to one another resulting in a coalesced-strand structure. These strand foams, however, are not used as core materials. Tenacious, tough thermoplastic resins such as polypropylene or polyethylene which generally exhibit lower stiffness, may be advantageously used for some applications, such as shock absorbers (see, e.g., U.S. Pat. No. 6,213,540 (issued Apr. 10, 2001)) but they offer poor performance as composite core materials for which high strength and stiffness are desirable.
In view of the above, there is a need for low-density closed-cell core materials that are rigid, strong, chemically and thermally resistant, and amenable to vacuum-mediated resin application.